It is proposed to describe the effects of internal and external free Ca ions concentrations on the kinetics of Ca ions release and ATP synthesis which characterize reversal of the calcium pump in sarcoplasmic reticulum vesicles isolated from mammalian skeletal muscle. Results obtained using a conventional, mixed population of vesicles will compared with those obtained using purified "light" and "heavy" fractions. No quantitative description is presently available for the effects of a wide range of intravesicular free Ca ions concentrations on the kinetics of reversal. It is known, that reversal is extremely rapid at the high internal concentrations of free Ca ions which can be obtained by loading vesicles with CaCl2 -- i.e., without Ca-precipitating anions. Experiments are designed to test whether effects of increasing internal free Ca ions may be attributed to an increased Ca ions concentration gradient, or to saturation of internal, low-affinity Ca-binding sites on the transport enzyme. They will show (1) how apparent affinities for substrates and products are affected by a range of well-defined levels of internal Ca ions, and (2) how internal Ca ions affects intermediate steps in the reaction sequence, as revealed by measurements of ATP-Pi exchange and phosphorylation. The role of external Ca ions will be analyzed similarly. Results will contribute to the understanding of how ion-protein interactions and transmembrane ionic gradients may drive ATP synthesis at the catalytic site of a membrane-bound ATPase.